In order to measure the camber and toe of the wheels of an automotive vehicle the common practice is to mount inclinometers to the wheels of the vehicle and to use these instruments to measure the deviation of the planes of the wheels from the horizontal and vertical planes. If the instruments are not precisely aligned with the axis of rotation of the respective wheels the camber and toe measurements will be inaccurate and thus result in misalignment of the wheels of the vehicle.
The inclinometers are generally mounted on a gauge head which is adapted to hang freely like a pendulum from a stub shaft carried by a wheel clamp mounted to the rim of the associated wheel. For various reasons the stub shaft will not be exactly coaxial with the axis of rotation of the associated wheel, a condition known as runout. Many ways of correcting for such misalignment or runout of the instruments are known and described in the literature.
For example, in U.S. Pat. No. 4,138,825 there is described a method of providing runout compensation by measuring and recording the runout at three different angular positions of the associated wheel while the wheel is elevated and then locking the instrument to the wheel to prevent relative movement between the instrument and the wheel and movement of the wheel from the final runout position before letting that wheel back down. A similar procedure is described in U.S. Pat. No. 3,892,042 where the runout measurements are made at two angular positions one-hundred eighty degrees apart before the instrument is locked to the wheel. In U.S. Pat. No. 4,192,074 still another method of compensating for the runout of the wheel alignment instrument is described. In this latter method the wheel is initially rotated through at least one complete revolution to measure and record the runout before the instrument is locked to the wheel and the various wheel alignment measurements are made.
It will be apparent that in all of these prior art systems it is necessary to lock the instruments to the wheels and prevent rotation of the wheel and gauge head combination after runout measurements have been made. Moreover, it is important that the relationships between the wheels and the associated instruments be maintained throughout the wheel alignment procedure or faulty measurements and a consequent misalignment of the vehicle will result. Because of the location of some of the devices which must be adjusted and in some cases because of carelessness, it is not unusual for the technician to accidentally bump one of the instruments and possibly cause it to move relative to the wheel to which it is mounted. When this occurs it is necessary to again jack up the vehicle and repeat the runout measurement procedure to ascertain if the instrument did indeed move relative to the wheel, whether it did in fact move.